This study evaluates the fitness of political parties for the democratisation of the European Union. At the national level political parties have successfully functioned as transmission belts between voter’s preferences and political outcomes in representative democracies. Some scholars have therefore argued that an increase in party competition at the European level could make the European Union more democratic; other scholars claim that European political parties are too weak to fend off public pressure, which would arise from an increase in political competition. Since cohesive voting behaviour of political parties is the basic prerequisite for a functioning representative democracy, this study analyses how the transnational party groups of the European Parliament are able to generate voting cohesion. Drawing on rational institutionalist theories of political parties and theories of collective action, the study outlines two competitive scenarios for explaining party group voting cohesion in the European Parliament. In the party group disciplinary scenario, the party group leadership is able to enforce voting cohesion through its disciplinary powers. The national party discipline scenario predicts that party group voting cohesion is dependent on the voluntary cooperation of the national party delegations. The empirical analysis of party disciplinary effects in the European Parliament corroborates the party group disciplinary scenario. The party group leadership of the two largest party groups is able to discipline it is members for disloyal voting behaviour. The findings do, however, also show that the party group leadership is not able to sanction national party delegations if they fail to toe the party group line. The study concludes that it will be difficult for the party groups to maintain voting cohesion, if public pressure on Members of the European Parliament increases through a more open form of political contestation at the European level. The responsibility for a successful democratisation of the European Union through party competition, therefore, lies in the hands of national political parties.